Apparatus for logging



Sept. 18, 1951 P. w. MARTIN APPARATUS FOR LOGGING s Sheets-Sheet 1 FiledNov. 8, 1944 v lnnento:

PHILIP W. MARTIN (Ittorneg Sept. 18, 1951 P. w. MARTIN 2,568,241

APPARATUS FOR LOGGING Filed Nov. 8, 1944 3 Sheets-Sheet 2 I AMPLIFIER 1:l a? I12 I85 Fig.4.-

17 g 6 K TIME I I37 I38 I35 I37 I39 SHALE I FRESH WATER f I35 I37 I40 mj I35 I37 SHELL- DENSE FORMATION I35 I37 I42 SALT WATER C I35 I37 I43SHALE I3 I44 OIL 35 I37' 45 I SALT WATER I3h I35.

Jnvenlor PHILIP W. MARTIN Gttorneg Sept. 18, 1951 P. w. MARTIN 2,568,241

APPARATUS FOR LOGGING Filed Nov. 8, 1944 3 Sheets-Sheet I5 ISnnentorPHILIP W. MARTIN (Ittorneg Patented Sept. 18, 1951 UNITED. STATES PATENTOFFICE APPARATUS FOR LOGGING "Philip W. Martin, Huntington Park, Calif.

Application November 8, 1944, Serial No. 562,502

1 Claim.

This invention relates to a method and apparatus for producing at thesurface of the earth a record or indication of variations in structureexisting in a subsurface strata, and is particularly applicable to'thelogging of deep wells such as oil wells. The primary object of theinvention is to provide an improved method and apparatus for logging theconditions and material existing in the various strata traversed by awell in the earth.

The invention may be used in the recording of resistivity,self-potential, temperature, pressure,

radio-activity, directional deviation of a drill hole, etc.

In the past it has been considered necessary I in connection withsuccessful methods of well logging to employ long wires or cables totransmit to the surface from the bottom of the hole information which ispicked up by exploring devices at the bottom of the hole. customary tomake the log after the hole is drilled. Under such conditions thepenetration of drilling mud and water into the formations traversed bythe hole greatly reduces the sharpness of definition of the conditionsoriginally existing in the formation'and introduces undesirablevariations of such conditions. Consequently, the log thus produced givesan indication of the conditions existing after the penetration of themud and water, but does not give a true indication of the conditionsexisting originally in the formation. It is therefore ex- It also hasbeen tremely desirable to produce the log during the drilling operation.Prior attempts to log during the drilling operation have been attendedwith such difiiculties as the necessity of measuring the average of agreat length of drill hole, making it impossible to achieve a sharpdefinition of conditions existing within thin strata such as, v

for example, oil sands which very often run as little as a few inches inthickness. Prior attempts to produce a log without the use of aninsulating conductor extending from the surface down to the point ofdrilling have failed because of the attempt to use a high frequencytransmitting wave. I have discovered that high frequency electricalimpulses are suppressed by the earths suppression value in the greatdistances that are required to be traversed between the bottom of a deepwell and the surface.

Having in mind these-inadequacies in the prior methods, m inventionprovides a method utilizing the transmission, from the bottom of a deepwell to the surface of the earth, of electrical impulses having afrequency below the suppression value of the earth between the bottom ofthe well and the surface, and thereby capable of reaching the surface ofthe earth and being there recorded.

A further object of the invention is to provide a method which makespossible the measurement of a condition of the earth at the bottom of adeep well by the transmission of electrical impulses from the bottom ofthe well to the surface, which impulses are varied in accordance withthe measurement taken at the bottom of the well, and the receiving ofthe impulses at the surface and the translation thereof intomeasurements corresponding to those taken at the bottom of the well.More specifically, the invention provides a simple and effective methodand apparatus whereby such transmission and reception is made possibleand practicable. To this end, the invention contemplates the utilizationof the measurements of a condition in the earths stratum being measured,to vary the frequency or duration of the electrical impulse which isbeing transmitted to the surface and the measurement, at the surface, ofsuch frequency or duration and the translation thereof back into anindication of the conditions at the bottom of the well. 7

Another object of the invention is to provide a method and apparatus forlogging which, in addition to the advantages specified above, is capableof simultaneously measuring and recording or indicating at the surfacetwo or more separate conditions existing at the bottom of the well. Tothe best of my knowledge, it has never before been proposed to transmit,upon an electrical wave or series of impulses traveling through theearth structure itself, separate indications of two or more differentconditions at the bottom of a well. The invention achieves this byutilizing the measure of the duration of an electrical impulse as ameasure of one condition and the measure of the interval betweenimpulses as the measure of another condition, and measuring thedurations of the impulses and the intervals between the impulses at thesurface so as to provide an indication or record of the conditions whichwere measured at the bottom of the well.

A further object of the invention is to provide an electro-responsiveapparatus at the surface of the earth adapted to respond, in two or moredifferent manners, to variations in the durations of, or polarity of,and the time intervals beexist in various strata throughout the lengthof a well, in combination with apparatus adapted to be utilizedsimultaneously during the drilling of the well to measure the two ormore conditions throughout the increasing depth of the well. and totransmit the measurements to the surface in the form of electricalimpulses capable of being picked up at the surface by said receivingapparatus and translated into said visual record.

Further objects and advantages of the inven- 4 the earth at the point Cof an electrical impulse (indicated at I9) which travels upwardly to thesurface and establishes a potential difference between the drill pipeand a selected point, indicated at D, in the surface of the earth at 'adistance from the hole It. This potential difference acts upon thetransforming unit I! tion will be brought out in the following part ofthe specification.

Referring to the drawings which are for illustrative purposes only:

Fig. 1 is a vertical sectional view through portions of a drill holehaving logging apparatus installed in connection therewith;

Fig. 2 is a wiring diagram of the apparatus of the transmitting unit:

Fig. 3 is a wiring diagram of the apparatus of the receiving unit;

Fig. 4 is a comparative graph showing an impulse in the form in which itis transmitted and the same impulse as modified by the suppressioneffect of the earth in traveling to the surface;

Fig. 5 is a comparative graph showing the modification, at the points oftransmission, by various earth formations, of the impulses transmittedby the invention;

Fig. 6 shows a section of a graph of a type produced by the invention;

Fig. 7 is a wiring diagram of a modified form of the receivingapparatus;

Fig. 8 shows the recording portion of the apparatus shown schematicallyin Fig. 7; and

Fig. 9 is a side elevation of the mechanism shown in Fig. 8.

As an example of one form of apparatu by which the invention may bepracticed, I have shown in Figs. 1, 2 and 3 a logging system capable ofcontinuously indicating during the operation of drilling a hole III foran oil well the self potential and resistivity of a measured section ofearth immediately adjacent the drill bit II carried by the lower end ofa string of drill pipe B, such as used in rotary method of well drillingwherein drilling mud. pumped-down through the drill pipe, passes upthrough the pipe. Using the drill pipe B as one conductor and the earthI3 as the other, the apparatus measures a difference of electricalpotential between the bottom of the hole where contact is made asindicated at A, and the average potential of the drill pipe. Thipotential difference is transformed into terms of time by electronicpick-up mechanism forming part of the transmitting unit II in the lowerend of the drill pipe B, andthe time period thus determined is, at thesurface, transformed back'into terms of the original condition by ameter I'I (e. g., a milliameter) of a transforming unit I8. This isaccomplished by starting, at the end of the time period thus determined,the transmission into (which is connected between the drill pipe andpoint D as shown) and operates to swing the needle of the meter I! to anextent determined by the length of the immediately preceding time periodduring which no impulse was being transmitted. The amplitude of swing ofthe meter needle depends upon the quantity of electrostatic chargestored in a condenser 20 (Fig. 3) arranged to discharge into the meterl1 when the transmission of the impulse It commences, is thusproportional .to the preceding time period during which the charge waspermitted to collect on the condenser plates and, accordingly, isproportional to the self potential between the point A and the drillpipe which determined such period of time.

In the succeeding period of time, during which the impulse I9 istransmitted, a charge is stored on a condenser 2| which, when theimpulse is terminated, discharges into a meter 22 to register a readingproportional to the length of such succeeding time period. Suchsucceeding time period is determined by the resistivity of the earthstratum between the points A and B, measured by a portion of theapparatus in the unit I8, and thus the reading on the meter 22 isdirectly indicative of such resistivity.

Having briefly outlined how the method is practiced, I will proceed todescribe in detail the illustrative form of apparatus shown in Figs. 1,2, and 3. The drill bit II may constitute an electrode for establishingelectrical contact at the point A. The drill bit II is connected to thestring of drill pipe B by a special section of drill pipe 24, which isconnected to the bit II by a short tube section I2 and insulatedtherefrom by a sleeve I4 of rubber or other insulating materialinterposed between the tube section I2 and drill pipe section 24, thelatter being covered by a sheath 25 of insulating material. A ring Ill,encircling the sheath 25 intermediate the ends of the section 24, is incontact, as indicated at C, with the body of water or drilling fluid 28which, since it fills the well around the drill pipe is normally presentin the bottom of a drilled hole, so as to provide a transmittingelectrode. The upper end of the transmitting unit It is grounded to thedrill pipe B by any suitable means, such as, for example, brushes 2'!attached to the casing of the transmitting unit, or simply by means ofcontact between the casing and the interior of the drill pipe. Thisestablishes a grounding connection to the drill pipe 13 of all of thevarious circuits of the transmitting mechanism, which will be explainedin detail hereinafter. Electrical connection of the transmitting unitwith the drill bit II maybe established by brushes 2B, suitablyinsulated from the casing of the transmitting unit, and contacting theinterior of the short drill pipe section I2 which is threaded onto thedrill bit. Electrical connection to the electrode I9I is established bybrushes 29 which are likewise suitably insulated from the casing of thetransmitting unit. The brushes 28 engage a ring I92 connected to thering I9I by suitable conducting member I93, the latter being insulatedfrom the pipe section I2 by a bushing I94. The brushes 2! and 28 extendthrough suitable aper- 2,ses,a41

tures or windows 88 in the casing of the transmitting unit Hi. It willbe understood that the transmitting unit is in the form of an elongatedcartridge having a diameter sufllciently small to freely pass throughthe string of drill pipe and having packed therein the various batteries(e. g., dry cells), thermionic tubes, and relays, etc., which comprisethe transmitting mechanism.

Referring now to Fig. 2, the measurement of earth potential involvesutilization of the difl'erence in earth potential between the point Aand the drill pipe B to establish a bias on the filament of a thermionictube 3| through a circuit including the bit electrode N (Fig. l), aconductor 82, a voltage divider 33, a conductor 34, contact 35 of arelay R, movable arm 35 of the relay R, a conductor 31, filament 38, andcontrol grid 38, which is grounded by a conductor 48 to the drill pipe B(a battery 4| provides current for heating the filament 38, and thescreen grid 42 is constantly biased with a positive charge from abattery 43) The earth potential is applied as a bias on the filamentcontrolling the electron flow from the filament to the plate 44 of thetube 3|. which gradually charges a condenser 45, making the platethereof which is connected to the tube plate 44 by a conductor 48, morenegative, and making the plate thereof, which is connected to thebattery 4| by a conductor 41, more positive.

The charging of the condenser 45 causes a positive bias to beestablished on the grid 48 of a gas triode 49, through a circuitcomprising the conductor 41, a second movable switch arm 5| of the relayR, a contact 52 of the relay R. a conductor 53, the grid 48, the cathode54, a conductor 55, a battery 58, a conductor l2 tapped to anintermediate plate of the battery 56, and the conductor 48. The positivebias thus established on the grid 48 induces a fiow of current from theplate 51 to the cathode 54, and when the difference in potential betweenthe cathode and grid reaches a predetermined voltage (for example, 90volts), the gas within the tube will ionize and establish a conductingpath from the plate 51 to the cathode 54, thence through the conductor55, the battery 55, a conductor 68 and a limiting resistance 59, andthence throughthe coil 58 of a relay R and back to the plate 51.

The energizing of the relay R, which is normaliy biased in openposition, closes the relay to establish a circuit for reversing therelay R and a circuit for energizing a vibrator V which cooperates withcondensers to form an alternating current generating means. The reversalof the relay R cuts out the earth potential measuring circuit andestablishes a transmitting circuit which impresses a sustained currentbetween the electrode C and the drill pipe. The reversal of the relay Ralso establishes a resistivity measuring circuit into which alternatingcurrent from the vibrator V is fed, to energize the measuring apparatus,and an electric impulse having a duration determined by this measuringcircuit. is transmitted at C into the earth from the transmittingcircuit. These circuits are as follows:

The vibrator is energized through a circuit including the movable arm 62of the relay R, the relay contact 83, a conductor 84, the vibrator coil85, a battery 58, and a conductor 81 which is grounded to the drill pipeB. When energized, the' vibrator produces impulses which, by condenseraction, are converted into alternating current in th generating circuitwhich includes the conductor '61, the battery 58, the movable vibratorarm 88. the fixed vibrator contact 88, a conductor 18, a condenser 1|,conductors 82 and 12, a condenser 13, a conductor 14, the plate and thefilament of a rectifying tube 18, and a bleeder re-- and battery 68.Energization of the coil 5| shiftsthe arm 85. to the contact 83, the arm5| to the contact 84', and a third movable arm 28 of therelay, to acontact l4. The latter connection establishes the transmitting circuit,including. the

conductor 18, the battery 18, and the conductor 88, which leads to theelectrode C. The potential of the battery 19 is impressed between theelectrode C and the drill pipe, to which the'relay arm 28 is grounded,for a short intervalof time determined by the resistivity measuringcircuit.

Resistivity is measured by rectifying the alternating current generatedby the actiorrot the vibrator V in the rectifier 18 and applying it,-through a resistance '85, a conductor 88, voltagedivider 81,.a conductor88, relay contacts 85 and 35, and the conductor 31, to the filament 58"of'the' amplifying tube 3|, thereby changing the'bias onthe tube. Whenthe vibrator contacts close, the

voltage 01' the battery 85 is impressed on thecohdensers 1| and 13,inducing a potential on time When the vibrator contacts rectifier tube15. open, the condensers discharge through a resistor 58 which isshunted across the vibrator. Potential is bled from the rectifier by theresistor 11. The earth provides a resistance between the electrode A andthe drill pipe B. The smaller thisresistance, the greater is the drainof the vibrator current to the earth and consequently lower therectified voltage rectified in the tub'e 18 and transmitted through aresistor 85, conductor 85, voltage divider 81, conductor 85, switchcontacts 88 and 35; and conductor 51 to the filament 38 of the amplifiertube 8|, thereby changing .the bias on the filament 58 to an extentproportional to the earth resistivity.

Thus biased, the tube 3| permits a gradual build up of charge on thecondenser 45 through the charging circuit previously described. Thischarge is applied to the grid "of a second gas triode 8| through a.circuit comprising the conductor 41, the relay arm 5|, the contact 84, aconductor 92, the grid 88, the cathode 58, the conductor 55, the battery58, and the conductors I2 and 48. When this bias reaches a predeterminedvoltage, the resistance of the tube 8| breaks down, the tube ionizes,and causes thetube 48 to deionize, thus opening the relay R, andterminating the transmitting cycle. Since the rate at which the bias onthe tube 5| is built up to the break-down voltage is dependentomresistivity, the duration of the transmitted impulse is a measure ofthe resistivity.

The deionization of the tube 48 is eflected by reducing the potential onthe plat 51 to a negative quantity. The plate |05'is connected to theconductor 88 by a resistor I81. A condenser I88 is 'bridged between theplates 51 and I88. when the tube 48 is conducting, the potential acrossit is much lower than the voltage applied by the-battery 58. Forexample, if the battery E. M. F. is 288 volts, the voltage drop throughthe resistor 58 and the coil 58 may be such as to place a potential ofonly'lO volts on the plate 51 and, consequently, on the left side of thecondenser I88, while the full voltage of 200 volts is applied to therightside of the condenser. When the tube 8| ionizes, the potential onthe right side of the condenser is dropped to zero and a corre earthpotential cycle, and is deionized when the tube 43 is ionized, in amanner similar to that described above.

Voltage dividers 33 and 31 function to adjust the initial fixed bias onthe filament 33. E. M. F..

forthis bias, is supplied by the batteries I09.

The resistor 85 and the condenser IIII form a filter. Contacts 35 and 83are grounded through condensers II I. which, in conjunction with thevoltage dividers 33 and 31, forms a filter which, with the filter 85,III), disposes of transient voltages such as those caused by opening andclosing.

of the relay contacts, etc.

The limiting resistors 53 and IIiI function to protect the gas triodes49 and 9i and the battery 56 against excessive current flow under thehigh voltage of the battery 56.

The pulses transmitted from the bottom of the well are received by anamplifier II2, Fig. 3 (a high gain amplifier with proper filters toeliminate commercial frequencies and other undesirable currents), whichamplifies them suillciently to energize the coil I85 of the relay R".The relay R has a pair of movable switch arms H3 and H4 cooperating withpairs of fixed contacts H3 and II! and H8 and H3 respectively. The relayis biased to engage the contacts II! and H9 during the intervals betweenpulses, and with these contacts engaged, the condenser 20 is chargedfrom a pentode voltage amplifier tube I20 through a circuit, including aconductor I2I, the relay, a conductor I22, the tube I20, the cathodelead I23, the B battery I24, and a conductor I25. When the transmittedpulse is received, the relay reverses, establishing a circuit throughwhich the condenser 20 discharges into the meter II to give a readingcommensurate with the charging time (the non-transmitting intervalpreceding the pulse) and thus is directly indicative of the earthpotential. The condenser discharge circuit includes the conductor I2I,relay contacts H3 and H6, a conductor I26, meter I1, and conductor I25.For the duration of the transmitted pulse the condenser 2| is chargedfrom a tube I21 (similar to the tube I20) through a circuit includingthe conductor I28, relay contacts H4 and H8, conductor I29, the tubeI21, the cathode lead I23, B battery I24 and the conductor I25.

At the end of the transmitted impulse, the relay is deenergized andreverses to its biased position, discharging condenser 2i into the meter22 to give a reading corresponding to the duration of the pulse and thusis directly indicative of earth resistivity. This is accomplishedthrough a discharge circuit including the conductor I28, relay contactsII4 and H9, the conductor I30, the meter 22, and the conductor I25.

Fig. 4 shows, in diagram form, a pair of oscillograms actually recordedphotographically from the screen of an oscillograph and produced by asignal transmitted from a depth of over 2000 feet, the lower osciliogramrepresenting the sustained pulse at the transmitting point, with thepotential rising instantaneously, as at I3I, from' zero to fullintensity which is continued as at I32 for the duration of thetransmitting period, then instantaneously dropping back to zero as atI33. The received pulse requires, however, a substantial time interval t(which at such depth may be about .06 second) to build up. as indicatedat I34. to full potential. which is Sustained as indicated at I35 forsubstantially the same period as the transmission period, and requiresanother interval t of about the same time as the buildup period, asindicated at I35, to decay back to zero. Oil wells are drilled in earthcomprising one or more wet formations, such as water bearing sands orother moisture carrying layers of minerals. These formations thereforehave a condenser effect which cooperates with the inductance and/orchoke effects of the earth structure to form filters which suppress allfrequencies except those within a very low range. To illustrate whatoccurs, I have in Fig. 4 shown time intervals t and t which may bereferred to as the suppression time characteristic of the formation orearth between the point of energization in the well and the surface ofthe earth. After the direct current flow or impulse is initiated in thewell as indicated at I3I there is a delay t during which time thecondensers formed by the formations are being charged and thecounter-inductances and choke eifect of the formations are beingovercome, with the result that at the surface the peak of the receivedwave indicated at I35 is not reached until after the suppression time orinterval t has elapsed, such time t for the formation traversed by the2000-foot depth of well recited in the foregoing being about 1*; of asecond. As shown by an oscillograph the energizing direct currentpotential is discontinued in the well as indicated at I33, theoscillograph shows that it takes the time interval t for the formationcondensers, restrained by the choke or self-potential effects of theformation, to discharge. Therefore, as a result of this condition, it isnecessary that the frequency of the direct current impulses be notsubstantially greater than about ten complete cycles per second, itbeing understood that the term cycle" refers to the electrical impulseand the time interval following it, or the time between successiveimpulses of the same polarity, regardless of whether these impulses ofthe same polarity are separated by intervals of reduced or zeropotential, by impulses of opposite potential, or by frequencies whichwill be suppressed by the formations.

Fig. 5 shows a set of curves indicating how the frequency of the pulsesand the intervals between pulses vary to indicate different earthconditions. The curves, designated I38, I39, I40. I4I, I42, I43, I44,and I45, correspond respectively to layers of shale, fresh water, shale,dense shale formation, salt water, shale, oil and salt water. In eachcurve, the pulses, measuring the resistivity, are indicated at I35 whilethe intervals, measuring the earth potential, are indicated at I31.

, Fig. 6 shows a pair of curves (I3la indicating earth potential andI35a indicating resistivity) nili plotted from a series of readingstaken con tinuously during a drilling operation. These curves indicatelow potential and low resistivity for shale, medium potential and highresistivity for fresh water, low potential and high resistivity fordense shell formation, and high potential and resistivity for oil. Sucha pair of curves can be continuously recorded on a sheet by thereceiving mechanism shown in Figs. 7, 8, and 9.

The permanent record apparatus includes the high gain amplifier II2,arranged to pick up transmitted pulses between the points B and D, andthe relay R3, having a movable arm I biased to engage a contact I5Iduring the interval between pulses and to establish a circuit throughthe conductor I52, the winding I53 of a reversible motor I54, theconductor I55, the battery I56. and the conductor Ill. when a il -i188is rmivld by amplifier H2, it is amplified to energize the relay R3,swinging the relay R3, swinging the relay arm I50 to engage the contactIIa and energizing the coil I53a of the motor I54, through the conductorI52a. On the shaft I58 of the motor (Fig. 9) are two potentiometerbrushes I60 and I60a, in engagement with potentiometer resistors I62 andI62a. Also, on the shaft I58 is an electrode arm I63 which sweeps fromside to side of a ribbon of sensitized paper I64 of the type adapted toregister a mark when an electric current is passed through it (such asthe paper known commercially as Teladeltos). On the opposite side of thepaper is a stationary horizontal bar electrode I65 which is grounded, asindicated at I66 in Fig. 9.

As the arm I63 swings in opposite directions, it is adapted to receivethe discharge of the condensers I61 and I61a, one side of each of whichis connected to the arm by a conductor I68, and to consequently placemarks on the paper I64. The spacing of these marks from the neutrallongitudinal axis of the paper is determined by the amounts of thecharges stored on the condensers I69 and I69a which have, during theperiods of time preceding the respective swings of the arm I63, receivedsuch charges, commensurate with the respective time periods, from tubesI10 and HM to which the condensers I69 and I 69a are connected byconductors HI and I1Ia. The charges on the condensers I69 and I69a areapplied, through conductors I12 and 112a, resistors I13 and I13a, andconductors I14 and H411, as a bias on the grids of gas tubes I16 andI16a, the plates of which are connected by conductors I11 and I11a tothe condensers I61 and I61a.

A biasing potential is applied to resistors I62 and IBM by a batteryI18. This potential is opposed, through the arms I60 and IBM, conductorsI80 and I80a, and resistors I82 and IBM, to the potentials of condensersI69 and "5911 respectively. As the arms I60 and I60a swing (in unisonwith the arm I63) they traverse the resistors I62 and I62a until they(alternately) reach points where the potentials applied from the batteryI18 are equal and opposite to those stored on the condensers I69 andIBM, whereupon the tubes I16 and [16a will in turn ionize and establishconducting paths including conductor I68, through which the condensersI61 and I61a discharge to the electrode I63, marking the paper I64 atpoints indicative respectively of the duration of the pulses andintervals, and thus being indicative of potential and resistivity in theearth formation being logged.

As the motor I54 completes its full amplitude of swing, a brush I83engages grounded contacts I84 and Na. in turn to short out thecondensers I69a and I69 to neutralize them for the next cycle ofoperation.

The drive for the paper I64 is synchronized with the drill pipe so thatthe paper is fed at a rate proportional to the rate of movement of thedrill pipe into the well.

The marks placed on the paper by the arm I63 will form curvescorresponding to those shown at I35a and I'31a in Fig. 6.

The above described circuit is a relaxation or trigger circuit,controllable by resistance or voltage changes. It furnishes two stablephases, and is capable of being modified so as to furnish additionalstable phases.

While the specific circuit described herein provides for transmission ofvoltages of one polarity by a discontinuous current or series ofunipolar impulses, it is to be understood that the invention may also becarried out by the use of a current of periodically reversing polarityand which may be either discontinuous or continuous. For example, therelay R could be modified so as to reverse the position of the battery19 in the circuit at the end of each phase of operation, therebyapplying an impulse of polarity opposite to that of the precedingimpulse, instead of cutting out the current fiow entirely.

I claim as my invention:

Apparatus for receiving a series of sustained electrical impulses ofappreciable duration which vary in accordance with variations intransmitting conditions, and for translating said impulses intoindications of the variations in the duration of said impulses,including: a two-position relay biased to one position in the intervalsbetween impulses and responsive to the impulses to assume its otherposition, a pair of electronic tubes and a means for producing a fiow ofcurrent therethrough, a pair of capacitances for storing the currentreceived from the respective tubes, 8. pair of meters adapted to measurethe quantity of E. M. F. stored in the respective capacitances, saidrelay being adapted, in one of its positions, to connect one of saidcapacitances to sai meter for said discharge of the capacitance chargeinto the meter to produce a reading thereon, and to interrupt theconducting circuit of one of said tubes and, in its other position, toconnect the other capacitance to the other meter for discharging anindicating current thereinto and to interrupt the conducting circuit ofthe other tube while reestablishing the conducting circuit of the firstmentioned tube and the connection thereof to the first mentionedcapacitance for charging the same.

PHILIP W. MARTIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,220,005 Rogers et al Mar. 20,1917 1,470,430 Ellison Oct. 9, 1923 1,966,224 Sommerfeld et al. July 10,1934 2,018,080 Martienssen Oct. 22, 1935 2,138,668 Stewart Nov. 29, 19382,145,026 Huxford Jan. 24, 1939 2,176,758 Borden Oct. 17, 1939 2,192,404Jakosky Mar. 5, 1940 2,295,738 Gillbergh Sept. 15, 1942 2,310,611Blondeau Feb. 9, 1943 2,314,873 Evjen Mar. 30, 943 2,336,929 Doyle Dec.14, 1943 2,354,887 Silverman et al Aug. 1, 1944 2,364,957 Douglas Dec.12, 1944 2,371,415 Tolson Mar. 13, 1945 2,377,757 Clark June 5, 9452,380,520 Hassler July 31, 1945 2,400,170 Silverman May 14, 19462,409,559 Haight Oct. 15, 1946 2,419,292 Shephard Apr. 22, 1949 OTHERREFERENCES Geophysical Exploration, Heiland, published 1940 byPrentice-Hall, Inc., N. Y. city. See page 723.

